Atomic displacements for the Si(111)7×7 reconstructed structure are calculated for five models which are consistent with scanning tunneling microscopy, an experiment in real space. They are (1) the adatom model, (2) the adatom π-bond model, (3) the adatom stacking-fault model, (4) the adatom stacking-fault π-bond model, and (5) the adatom double-stacking-fault model. The unreconstructed structures of these models do not explain very well the results of low-energy ion scattering spectroscopy, another experiment in real space. The reconstructed structures of all models give better results. All give similar spectra to those of the ‘‘pyramidal’’ model, which has been asserted to be the best explanation of experiment. The reconstructed adatom stacking-fault model gives perhaps the best spectra. However, at this stage, the best model really cannot be determined using only ion scattering spectroscopy, because the experimental spectra have broad peaks. It is hoped that further ion scattering work will show fine structure or that a third experiment in real space will be done in the near future.

• Received 28 January 1985

DOI:https://doi.org/10.1103/PhysRevB.32.2356